1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic method, such as a copier, a printer, or a facsimile machine, and in particular relates to an image forming apparatus capable of optimally correcting density fluctuations in an output image.
2. Description of the Related Art
An image forming apparatus employing an electrophotographic method forms an image by uniformly charging a photoreceptor or an image carrier by a charger, after which a latent image is formed on the photoreceptor by an exposure unit based on input image data and toner is adhered on the latent image by a developing device to thus render it a visual image. Such image forming apparatuses are widely used in the print industry and demand for ever higher quality is acute. To cope with such demand, high-speed image forming apparatuses have adopted various technologies.
Of those various quality requirements, uniform density over any given printed page is highly demanded and the uniformity in the printed page is a decision factor when a user selects an image forming apparatus. Fluctuation in the density within one page has various causes, such as unstable charge due to uneven charging; fluctuation of exposure by the exposure unit; variations in the sensitivity of the photoreceptor; variations in the resistance of a developing roller; fluctuation in the charge of the toner; and variations in the transferring of a transfer roller.
In the image forming apparatus employing the electrophotographic method, toner is deposited on the photoreceptor using an electrical field created by a potential difference between the developing roller or sleeve and the photoreceptor. It is generally known that the electrical field changes with distance. Specifically, when a developing gap fluctuates, the density also fluctuates. Because the density fluctuation caused by the rotary oscillation of the image carrier and the developing roller occurs cyclically and can be seen by the human eye, many customers raise claims for such a density fluctuation. In addition to the above factors, the density fluctuation from the oscillation of an intermediate transfer belt or uneven sensitivity of the photoreceptor varies, from the large cyclic density fluctuations to minute density fluctuations.
Various correction techniques have been proposed. Conventionally correction of the density fluctuation due to the rotary oscillation of the image carrier has been effective when the relative positions of the photoreceptor and the developing roller do not change from the time when the density fluctuation profile of one cycle of the photoreceptor is measured to correct the density fluctuation. However, when a print job is again executed after another print job has finished and the relative positions of the photoreceptor and the developing roller have changed, because the density fluctuation profile has changed, the density fluctuation cannot be corrected. Instead, a new density fluctuation occurs.
Even though the rotation cycles of the photoreceptor and the developing roller are set at an integral multiple of each other so that the density fluctuation profile becomes consistent. However, if the rotational speeds of the photoreceptor and the developing roller are different, the photoreceptor and the developing roller stop at different positions. Accordingly, it is difficult to keep the relative positions of the photoreceptor and the developing roller constant.
As a result, the density fluctuation cannot be prevented completely by simply measuring the density fluctuation and adjusting it with the rotational cycle of the photoreceptor, and a satisfactory correction effect cannot be obtained.
Herein, with reference to FIG. 20, a conventional density fluctuation correction method will now be described.
In the conventional density fluctuation correction method for correcting irregular rotary oscillation of the photoreceptor, it is first determined whether the density fluctuation correction is necessary or not. The necessity of the density fluctuation correction can be determined when the photoreceptor is replaced, when the photoreceptor detection position is shifted due to any reason, or optionally by the user mode. If it is determined that the density fluctuation correction is necessary, a detection pattern is formed so that the density fluctuation can be detected. The detection means in this case may be a density sensor or a density output on a sheet of paper. The detected density fluctuation data is averaged by the cycle of the photoreceptor, a phase and amplitude are adjusted so as to eliminate the density fluctuation, and the adjusted data is fed back as developing bias data. The fed-back developing bias is cyclically applied based on the relative positions of the developing roller and the photoreceptor. As described above, because the developing bias is corrected relative to the cycle of the photoreceptor, the density fluctuation due to the rotary oscillation of the photoreceptor is reduced.
FIGS. 21A and 21B are block diagrams each illustrating a device configuration for executing conventional density fluctuation correction.
As illustrated in FIG. 21A, a density fluctuation data storage includes reference density fluctuation data under specific image forming conditions. The density fluctuation data is data detected by a density sensor from an image previously formed by the image forming apparatus. Specifically, a data patch corresponding to 5 cycles of the photoreceptor is stored. A configuration in which data output on a sheet of paper is optically measured may also be used as density fluctuation data of the density fluctuation data storage.
A CPU converts the density fluctuation data of the storage into the correction data corresponding to the developing bias. The correction data is converted into analog signals by a D/A converter in synchronization with the photoreceptor rotary position detection signal, and the correction bias is applied to the developing roller from the developing bias high-voltage power supply so that the output image is controlled.
In a case in which the developing bias high-voltage power supply is PWM-controlled as illustrated in FIG. 21B, the correction data synchronizes with the photoreceptor rotary position detection signal and is PWM-controlled by the CPU. The correction bias is applied to the developing roller by the developing bias high-voltage power supply so that the output image is controlled.
FIG. 22 shows an example of the correction results executed by the conventional density fluctuation correction method. The vertical axis shows density fluctuation and a horizontal axis shows time elapsed in the photoreceptor rotation direction position. The white-out line shows a case in which the density fluctuation correction is not performed and the black line shows a case in which a correction is performed once.
JP-H09-62042-A discloses an image forming apparatus of the electrophotographic method or the electrostatic recording process for the purpose of exclusively reducing the stripe-shaped density fluctuation generated cyclically in the output image. The image forming apparatus disclosed includes a first fluctuation data storage to previously store the cyclical density fluctuations data of the image density; and a first controller to control the image forming condition based on the density fluctuations data, in which the first fluctuation data storage stores at least the density fluctuations data corresponding to one cycle of the developer carrier, and the first controller controls at least one of the charged voltage, the exposure light amount, the developer voltage, and the transfer voltage, whereby the density is corrected by the controller in accordance with the rotation cycle of the image carrier.
However, the conventional technology as described above cannot satisfactorily resolve the problem of density fluctuation due to the variation in the rotational cycle of the image carrier.